In order to measure the dimensions of a profile of an object, a range camera may be used. A range camera typically comprises a light source and a sensor wherein the light source is adapted to produce a light plane on the object to be measured. Furthermore, optics is generally located between the sensor and the object for focusing light reflected from the object onto the sensor. The light source, the object and the sensor are generally located at a distance from one another such that they each form a corner of an imaginary triangle.
The sensor extends in a sensor plane and as may be realized by a person skilled in the art, in order to be able to determine the dimensions of the profile, there is a need for a mapping from points in the sensor plane to points in the light plane such that coordinates in the sensor planes may be translated to real world coordinates. The process of obtaining such a mapping is generally referred to as a calibration of the range camera. Due to inter alia an unknown scale of the mapping, an unknown perspective distortion of the light plane in relation to the sensor and an unknown distortion of the aforementioned optics, such a calibration is generally determined by means of measurements of a reference object.
To this end, prior art proposes various ways of performing the aforesaid calibration. For instance, a document by Z. Zhang named “A flexible new technique for camera calibration.” IEEE Transactions on Pattern Analysis and Machine Intelligence, 22(11):1330-1334, 2000, proposes that a two-dimensional pattern, such as a checkerboard, is placed in the presumed extension of the light plane, although the light source has been turned off. The pattern may be placed in at least two positions, wherein at least one of the positions is in the extension of the light plane, such that a calibration may be performed. However, since the light source has been turned off during the calibration procedure as proposed hereinabove, there is of course a risk that the pattern is unintentionally positioned outside the extension of the light plane, which will impair the result of the calibration. Moreover, the aforesaid method is based on the assumption that the light plane produced by the light source is perfectly planar, which is not always necessarily true, and this assumption may also impair the result of the calibration procedure.
Alternatively, prior art teaches that a calibration object may be placed in a plurality of predetermined positions in the light plane and a mapping from the sensor plane to the light plane is performed by using images of the plurality of positions as well as information as regards the aforesaid positions. However, such a calibration procedure requires that the positions of the calibration object may be determined appropriately accurately, which generally results in that the positioning of the calibration object is performed by using a motion rig. Apart from being expensive and cumbersome to use, the motion rig also has the disadvantage that it requires space around the light plane, which space is not always available due to e.g. space limitations close to the range camera.
As may be realized from the above, there is a need for further improvements of calibration procedures for range cameras which removes at least one of the disadvantages of the prior art calibration procedures defined hereinabove.